Radar Reflectivity: March 1, 2017 Severe Weather Event

March 1, 2017 Severe Weather Event

Strong linear storms, sparked by an incoming cold front, swept across most of the eastern united states. There was also strong forcing along the impending cold front, as shown in figure 1. Due to the very warm temperatures prior to the cold front with steep mid-level lapse rates and some wind shear, that encouraged the storms to become severe. However, the Georgia area had enough instability for convective development which gave way to the watches and warnings which were issued throughout the day from 11:50 AM to about 9:00 PM, as you can see in figure 2. Strong winds downed many trees and power lines, leaving roughly 32,000 people without power. Meanwhile, further north, the great smoky mountains national park, straddling Tennessee and North Carolina, closed all roads due to the high wind danger. Many reports of hail were also recorded and even one brief tornado in northern Georgia. The QLCS (quasi-linear convective system) tornado had 90 MPH winds, the EF-1 tornado touched down near Chatsworth, GA, tracking three miles west of Chatsworth. As you can see in figure 3, the cell to the northeast of the red point (Chatsworth, GA) seems to be the one from which produced a tornado. Unfortunately, the closest radar, KHTC in Hunstville/Hytop, AL, was down due to the power outages thus the next closest radar was used, KFFC in Atlanta, GA. The NWS (national weather service), from KFFC, provided figure 4 which shows the storm when it produced the EF-1 tornado over Chatsworth, GA at about 4:30 PM. This system also produced up to golfball sized hail and peeled back roofs. However, as the system continued to move southeast into central Georgia, the storms weakened substantially. Nonetheless, before dying out, this system produced roughly five dozen tornadoes, over 600 high wind reports, and over 100 large hail reports as it tore through parts of the Midwest, South and East United Stated from February 28 to March 1, 2017, making it the largest severe weather outbreak since late spring 2011.

Figure 1

Figure 2

Figure 3

Figure 4

Velocity Radar: November 5, 2017 KBUF

November 6, 2017—Buffalo, NY

A storm system moved through the area of Buffalo, New York on November 6, 2017 between 0029Z and 0200Z. A cold front was prominent near Saint Catharines, Ontario, Canada to Erie, Pennsylvania and crossed the region from west to east between 0100Z and sunrise. In advance of this cold front, there was moderately strong south-southwest low-level flow with surface gusts up to 30 MPH (fig.1). In radar imagery from GIF1 and GIF2, there seems to be a mesoscale convective vortex (MCV) present over northwest Pennsylvania. Towards the beginning of GIF1, there was actually an EF-1 tornado reported at 23:22 UTC (6:05 PM EST) by the National Weather Service (NWS) storm survey team, just four miles southwest of the town of Erie in the Millcreek Township. The tornado track was roughly 2.4 miles in length by 100 yards in width, with maximum winds estimated to be 90 MPH. Fortunately, no one was killed or injured during this event that brought nearly three inches of rain to the area. However, there was not really any indication on radar that there was a tornado. Next to the green inbound winds, there was no red outbound winds near the area, instead there was purple haze, indicating that the radar was unable to determine the wind’s velocity, which is referred to as range folding (RF) or velocity folding. Depending on the radar’s operation mode and PRF (pulse repetition frequency), the range folded data may occasionally obscure large portions of the radar’s image. As you may notice in GIF3, there seems to be a decent amount of range folding near the southern end of this storm system. At any rate, another reason I would not have thought there was a tornado just from radar alone was due to the lack of intensity of the inbound winds. Generally, intense circulations with diameter of one mile or less are tornadic, while large couplets are associated with mesocyclones. There is no specific value of diameter and magnitude that differentiates between the two due to the variety of circulations that may occur and due to gates having a variety of volumes and height, which depend on range and beam width. However, in this case, I suspect that the radar probably overshot the tornado circulation completely, given that the tornado occurred roughly 8,351 feet from the radar.

Figure 1

GIF 1

GIF 2

Figure 2

GIF 3

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Radar

Ø  Radar (radio detection and ranging):

̶        Sends out a signal .95/.96 degree beam width (older radar had a 2.0 degree beam width) that are trying to detect precipitation when it turns itself off to listen for an echo.

̶        Radar hits a raindrop, when it hits the raindrop the radar beam scatters, some of it comes back

̶        Not able to resolve/resolution the details when the storms are far away with one beam but when you are close and you can distinguish with more than one beam

̶        Decreased beam width=decreased resolution.

̶        Parabola dish.

Speed of light * time = distance ÷ 2

̶        Beam: Arches/normal refraction (not linear) – the way density changes with height is what makes the radar arch more or less

̶        Smaller pulse length = better resolution = better sensitivity

Low-level Rotation and Storm Top Divergence

̶        Each box is at different heights in the atmosphere happening at the same time

1.    Shows rotation

2.    ____

3.    Rotation, a little divergence, Strong asimuthial shear, faster in, faster out – TVS

4.    Radial shear, divergence

̶        Radial Shear: Traveling along a radius (convergence or divergence)

̶        Radial Velocity: component of the actual velocity moving towards or away from the radar

̶        Azimuth angle: the angle the radar is at

̶        Volume Coverage Pattern (VCP): Telling the radar how to operate (scanning at a certain height, then raise the beam and scan again, then raise the beam and scan again…)

̶        Algorithms: algorithms that tells the radar to do something

Ø  Azimuth Resolution Considerations

̶        The further something is from the radar the less intense the rotation may seem

̶        When the tornado is too small or too far from the radar = resolution is not fine enough = get a closer radar or a smaller beam

̶        You have to use other things that the tornado is there, like the mesocyclone

̶        FAR – False Alarm Ratio – Better to have a false alarm then no alarm when a tornado is there

̶        An event is warned for but does not occur results in a false alarm

̶        POD – Probability of Detection -  Ratio of how many times I got a warning to the amount of events (Something is there and I have warning out for it)

̶        An event that occurs and has been warned for results in a 100% POD

̶        We want FAR down (40%) and POD up (70%)

̶        FAR and POD can be used for either Thunderstorms or Tornados

̶        CSI – Critical Success Index

Ø  SRV vs. Base Velocity with Subtle Rotation

̶        When I want to know if there is damaging winds on the ground or rotation, what’s going on, on the ground?

Base Velocity	Storm Relative
When diagnosing straight line winds use base velocity	When diagnosing rotation, use storm relative velocity
The strength of an advancing line of storms producing straight line winds is the sum of the winds produced by the storms, plus the movement of the storms.	SRV subtracts out the motion of a storm to display pure rotational characteristics of that storm.
How fast winds actually are	How fast something is rotating
Use: Actual Wind	Use: Rotation

̶        Rmax (unambiguous [clear] range): The furthest distance the beam can travel away from the dish and back before the next beam is sent out – c / (2 * PRF)

̶        Range Folding: Radar displaying and echo 1 Rmax closer (ping pong balls)

̶        PRF (pulse repetition frequency): how often a pulse is sent out

̶        Vmax (velocity Interval):

o   The faster something is moving, then I need more samples to measure it accurately (PRF)

o   As Vmax goes up, Rmax goes down (Doppler Dilemma)

̶        Doppler Dilemma: There is no single PRF that maximizes both Rmax and Vmax

o   High PRF’s = short unambiguous ranges and vice versa

o   Low PRF’s = velocity aliasing and vice versa

̶        How does a Doppler radar determine if an object is going towards or away from the radar?

o   The shift in frequency determines whether an object is moving toward or away from the radar

o   Frequency of what? Wave length/Radio waves

̶        Red Shift: Everything is moving away from each other (Big Bang)

̶        Bigger Rmax = Less Range Folding

o   How do I get a large Rmax?

̶        Aliasing:  Bad velocity data (wall paper example) “fold over”

̶        Isodop: “S” shaped, winds are veering with height (hurricane Katrina)

̶        Veering (VW – Veering warm): Turning clockwise with height

̶        Backing (BC – Backing Cold): Counter-clockwise

̶        TBSS (Three-Body Scatter Spike):

o   Beam hits the stone = some comes back, some scatters = hits ground, comes back to hail = returns to radar

o   Radar thinks it’s further away because it takes longer to return (flare)

o   Best indication of large hail (~1.5in. diameter hail)

̶        Stone: Hail stone

̶        Flare: nothing is there, not real

̶        AP (Anomalous Propogation): a low-level inversion created by the cold pool results in superfraction and thus AP

Ø  Reflectivity:

̶        dB: decibel – 10 log (power returned / reflected power)

̶        Hail: Big raindrop on the radar

̶        m: Milliwatt (thousandth)

̶        D: Change in diameter

̶        Z:

o   is reflectivity (of a single raindrop)

o   = D^6 (64 times more power back)

o   Proportional to D⁶

̶        Log: 10^0=1   -   Log1=0

̶        Reference Power: The amount of power you get back with a 1millimeter raindrop per 1cubic meter space – changes based on the distance to the radar

̶        dBZ: decibel of recent activity/reflectivity – 10 log (power / reflected power)

o   0 dBZ = 0

̶        dBm: measuring the power of the return – 10 log (power returned / 1 milliwatt)

o   0 dBm = 1 Milliwatt

̶        Size of raindrops determine if there will be a tornado

Every time I double the power, I add 3: Doubling of power results in a linear increase of 3dBZ

̶        Sends out a signal (750,000 watts)

̶        Double the size of a raindrop = gives back D⁶

   30dBZ

+ 3(six times)

   48dBZ

Ø  VCP (Volume Coverage Patterns)

̶        Clear Air Mode:

o   Want to be sensitive

o   Longer pulses = Higher sensitivity

Ø  Doppler

̶        Radial Velocity: Velocity toward or away from the radar, shifts in the frequency

o   Toward: higher frequency

o   Away: lower frequency

̶        Frequency of Sound: pitch

̶        Doppler Shift/Effect: frequency of radar energy caused by the movement of precipitation or other objects in the radar beam toward or away from the radar

̶        Vector in the Components: (ping pong balls with someone walking)

Ø  NIDS - Nexrad Information Dissemination Service

̶        Dissemination: Distribute information

̶        Base Velocity (storm relative):

o   Various elevation angles

o   When diagnosing Straight Line Winds (bow echo, derecho, microburst’s)

̶        VIL (Vertically Integrated Liquid): Sum up how much liquid is in a storm

̶        VWP (VAD Wind Profile)

̶        VAD (Velocity Azimuth Display)

̶        Composite Reflectivity: “here’s the big storms” – all on the same display, doesn’t care where in the storm is strong or weak

̶        Radial Velocity: Velocity toward or away from the radar, shifts in the frequency

o   Toward: higher frequency

o   Away: lower frequency

Ø  Interpreting Doppler Radar

̶        Zero Isodop: winds are perpendicular from green to red, looks like an “S” on the wind display, veering winds with height = warm air advection = rising air

̶        Veering: Clockwise shifting

̶        Advection: Horizontal movement of air

Ø  Backwards “S”: backing winds with height = cold air advection = sinking air (subsidence)

Ø  Blow from green to red

Ø  Duel-Polarization Radars

̶        http://wdtb.noaa.gov/courses/dualpol/outreach/non-mets-intro/player.html

̶        Hole = debris from tornado